Iron is known to be the most common and at the same time most detrimental impurity in aluminium alloys since it causes hard and brittle iron-rich intermetallic phases to precipitate during soidification. The most detrimental phase in the microstructure is the beta-phase of the Al.sub.5 FeSi-type because it is platlet-shape. Since the detrimental effect increases with increasing volume fraction of the beta-phase much interest has focused on the possibilites of reducing the formation of said phase, as recently reviewed by P. N. Crepeau in the 1995 AFS Casting Congress, Kansas City, Mo., 23-26 April 1995.
The problem related to iron contamination of alumninium alloys is of great economical interest since 85% of all foundry allous are produced from scrap, the recycling rate is ever increasing (already higher than 72%) and the service life of aluminum is relatively short (of about 14 years). As a result thereof, the iron content in aluminium scrap continouosly increases since iron cannot be economically removed from aluminium. Dilution is the only practical method to reduce the iron content and the cost of aluminium is known to be inversely related to its Fe content. On the other hand, iron is deliberately added in an amount of 0.6-2% to a number of die-casting alloys, eg BS 1490: LM5, LM9, LM20 and LM24. Moreover, due to the low diffusivity of iron in solid aluminium there exist no practical possibility to reduce the deleterious effect of the iron containing precipitates by a heat treatment.
Iron has a large solubilty in liquid aluminium but a very low solubilty in solid aluminium. Since the partition ratio for Fe is quite low, iron will segregate during solidification and cause beta-phase to form also at relatively low iron contents as shown by Backerud et al in "Solidification Characteristics of Aluminium Alloys", Vol. 2, AFS/Skanaluminium, 1990. In said book the composition and morphology of iron containing intermetalic phases are detailed in relation to the Al-Fe-Mn-Si system.
The two main types occuring in Al-Si foundry alloys are the Al.sub.5 FeSi-type phase and the Al.sub.15 Fe.sub.3 Si.sub.2 -type phase. Moreover, a phase of the Al.sub.8 Fe.sub.2 Si-type may form. These intermetallic phases need not be stoichiometric phases, they may have some variation in composition and also include additional elements such as Mn and Cu. In particular Al.sub.15 Fe.sub.3 Si.sub.2 may contain substantial amounts of Mn and Cu and could therefore be represented by the formula (Al,Cu).sub.15 (Fe,Mn).sub.3 Si.sub.2.
However, for typing reasons the simplified formulas Al.sub.15 Fe.sub.3 Si.sub.2, Al.sub.8 Fe.sub.2 Si and Al.sub.5 FeSi are preferred in the following. Accordingly, it is to be understood that compositional and stoichoimetrical deviations of the phases at issue are covered by the simplified formulas.
The Al.sub.5 FeSi-type phase, or beta-phase, has a monoclinic crystal structure, a plate like morphology and is brittle. The platlets may have an extension of several millimeters and appear as needles in micrographic sections.
The Al.sub.8 Fe.sub.2 Si-type phase has a hexagonal crystal structure and depending on the precipitation conditions this phase may have a faceted, spheroidal or dendritic morphology.
The Al.sub.15 Fe.sub.3 Si.sub.2 -type phase (often named alpha-phase), has a cubic crystal structure and a compact morphology, mainly of the chinese script form.
In the Al-Fe-Mn-Si system these three phases have been represented in the Si-FeAl.sub.3 -MnAl.sub.6 -equilibrium phase diagram as described by Mondolfo, FIG. 1. It may be noted that the Al.sub.15 Fe.sub.3 Si.sub.2 -type intermetallic is denoted (Fe,Mn).sub.3 Si.sub.2 Al.sub.15 in this figure. Point A represents the composition of a foundry alloy of the conventional A380-type and it can be seen that its original composition lies within the (Fe,Mn).sub.3 Si.sub.2 Al.sub.15 area. The solidification of such an alloy typically starts with the precipitation of aluminium dendrites and, in course of the solidifcation, the interdendritic liquid becomes sucessivley enriched in iron and silicon. As a result, the Al.sub.15 Fe.sub.3 Si.sub.2 -type intermetallic phase starts to precipitate (represented as(Fe,Mn).sub.3 Si.sub.2 Al.sub.15 in this diagram). Fe and Mn are consumed due to this reaction. The liquid moves towards the Al.sub.5 FeSi-area and starts to co-precipitate large platelets of Al.sub.5 FeSi-type phase until the liquid composition reaches the eutectic composition at point M in the phase diagram where the main eutectic reaction take place. For further details on the solidification of commersial aluminium foundry alloys, reference is given to Backerud et al, "Solidification Characteristics of Aluminium Alloys", Vol. 2, Foundry alloys, AFS/Skanaluminium, 1990.
As already pointed out, the primary platelet-shaped beta-phase of the Al.sub.5 FeSi-type is the most detrimental iron containing intermetallic phase in aluminium alloys because of its morphology. The large beta-phase platelets have been reported to decrease: ductility, elongation, impact strength, tensile strenght, dynamic fracture thoughness and impact thoughness. The effect has been attributed to: easier void formation, cracking of the platelets and microporosity caused by the large beta-phase platelets. In addition, the coarse beta-phase platelets have been reported to infer with feeding and castability and thereby increase the porosity. The perhaps most important effect of the platelets for many industrial applications is that they give rise to microporosity which is the most likely source of crack initiation.
In summary, it can be concluded that increased Fe may result in unexpected formation of the deleterios platelet-shaped beta-phase. The beta-phase forms above a critical iron content, causing the mechanical properties to decrease drastically.
Accordingly, in the prior art much work has been directed to the possibilites of avoiding the formation of beta-phase.
Prior art methods for reducing the formation of beta-phase can be grouped into the following four classes:
1. Control of Fe-content.
2. Physical removal of Fe.
3. Chemical neutralization.
4. Thermal interaction
The first method is based on careful control and selection of the raw materials used (ie low-Fe scrap) or dilution with pure primary aluminium. This method is very costly and restricts the use of recycled aluminium.
The second method relates to sweat melting and sedimentation of iron rich intermetallic phases by the so called sludge. However, both methods result in considerable aluminium losses (about 10%) and are therefore economically unacceptable.
Chemical neutralization is, so far, the most used technique. Chemical neutralization aims at inhibit the platelet morphology by promoting the precipitation of the Al.sub.15 Fe.sub.3 Si.sub.2 -type phase which has a chinese script morphology by the addition of a neutralizing element. In the past, most work has been directed to use of the elements Mn, Cr, Co and Be. However, these additions have only been sucessful to a limited extent. Mn is the most frequently used element and it is common to specify % Mn&gt;0.5(% Fe). However, the amount of Mn needed to neutralize Fe is not well established and beta-phase platelets may occur even when % Mn&gt;% Fe. This method can be used to suppress the formation of beta-phase. However, it is to be noted that the total amount of iron containing intermetallic particles increases with increasing amount of manganese added. Creapeau has estimated that 3.3 vol. % intermetallic form for each weight percent of total (% Fe+% Mn+% Cr) with a corresponding decrease in ductility. In addition, large amounts of Mn are costly. Chromium and Co have been been reported to act similar as Mn and both elements suffer from the same drawbacks as Mn. Beryllium works in another way in that it combines with iron to form Al.sub.4 Fe.sub.2 Be.sub.5, but additions &gt;0.4% Be are required which causes high costs in addition to the safety problems related to the handling of Be since it is a toxic element.
The last method--thermal interaction--can be performed in two ways. Firstly, by overheating the melt prior to casting in order to reduce nucleating particles that form the detrimental phases. However, hydrogen and oxide contents increases, process time is consumed and costs are incurred. The second possibility is to increase the cooling rate in the combination with an addition of Mn. By increasing the cooling rate the amount of Mn needed decreases somewhat. Although this technique limits the drawbacks of the chemical neutralization by Mn it may be hard or impossible to put into practice in commercial foundry production, in particular for conventional casting in sand moulds and permanent moulds with sand cores.
Accordingly, the object of this invention is to propose an alternative method to avoid the formation of the deleterious plate like beta-phase in iron containing aluminium alloys. In particular, it is an object to propose a method which does not suffer from the above mentioned problems.